Conventionally, from the viewpoint of wave-band constitution and chemical stability, in the field of the III-V family nitride semiconductors consisting of the III family elements such as aluminum (Al), gallium (Ga), and indium (In), and of nitrogen (N) corresponding to one of the V family elements (this will merely be referred to as a GaN composite semiconductor hereinafter), there has been much expectation on the potential utility of this GaN composite semiconductor for its practical application to a light emitting element and a power device or the like. In order to realize the potential utility of this semiconductor, much effort has thus been pursued so as to manufacture a nitride composite semiconductor laser element capable of emitting blue laser beam, for example, by way of laminating the GaN composite semiconductor layers on a sapphire substrate (composed of Al2O3) or on a SiC composite substrate.
Nevertheless, when a thin film consisting of a crystalline GaN composite element has been formed on a sapphire substrate or on a SiC composite substrate, due to difference in the lattice constant between the GaN composite element and the substrate material, various defective phenomena such as “dislocation” for example are induced into an epitaxial layer. Because of this reason, in the case of a nitride composite semiconductor laser element enabling high-density current to flow through it, the above-cited defect triggers so as to cause the lattice constitution to become disordered, thereby further causing the above defective phenomena to potentially propagate themselves. Further, when the nitride-composite semiconductor laser element has been formed on a sapphire substrate, there is a problem in terms of its practical service life. It is conceived that, this is probably because occurrence of the high-density dislocation in the above element adversely acts, thereby restricting potential service life of the nitride-composite semiconductor laser element.
Due to the above reason, ideally, any substrate usable for loading the above GaN composite semiconductor device should preferably be composed of mono-crystalline GaN composite material. This constitution eliminates the difference of the lattice constant between the substrate and individual layers laminated on the substrate. Further, because the GaN composite compound itself contains compatibility with cleavages, a process for cutting a wafer into a plurality of chips can be facilitated. Further, since the GaN composite crystals are electrically conductive, processes for disposing electrodes in the above device can be simplified. Owing to these advantages, mono-crystalline GaN composite material is conceivably most suitable for constituting a substrate.
There is a report on the result of development in the field of a nitride composite semiconductor laser element capable of oscillating itself in a range from the ultra-violet region up to the visible rays region as shown in “Jpn. J. Appl. Phys. Vol. 39 (2000)”, on pages L647 to 650. The nitride composite semiconductor in subject features a constitution in which a SiO2 mask pattern with periodically striped openings was formed on a GaN composite substrate, and further, a structure consisting of laminated nitride semiconductor layers with a striped wave-guide passage (i.e., a ridge stripe structure) was formed on the above-cited mask pattern.
The above-cited GaN composite substrate was manufactured via execution of a process described below. By applying a MOCVD (Metalorganic Chemical Vapor-phase Deposition) method, a GaN composite layer with 15 μm of thickness was formed on the grounding seed crystals superficially being provided with a SiO2 mask pattern containing striped openings per 20 μm of cyclic period before eventually generating a wafer with a flat surface. This method is conventionally defined as the ELOG (Epitaxially Lateral Overgrown) method, which, by way of availing lateral growth, causes defective phenomena to be decreased. Further, by applying a conventional HVPE (Hydride Vapor Phase Epitaxy) method, a GaN composite layer with 200 μm of thickness was formed, and then the grounding seed crystals were removed before eventually completing the processes for manufacturing a GaN composite substrate. In terms of the service life characteristics of the produced semiconductor laser element, it was so estimated that a maximum of 15000 consecutive service hours could be available under 30 mW of an output condition at 60° C. of environmental temperature.
Nevertheless, in the case of the above-cited nitride semiconductor laser element, since the method for manufacturing the GaN composite substrate necessitated growth of crystals by three cycles including the above-referred HVPE method, the MOCVD method for growing the grounding seed crystals, and the other MOCVD method for growing structure of semiconductor laser element, the method of manufacturing the above-cited nitride composite semiconductor laser element involves much complexity, thus generating a problem in terms of the productive efficiency. Further, the service life characteristics were evaluated to be still not sufficient. In particular, the service life characteristics were still insufficient under a high output condition (at 70° C. and 60 mW for example). In addition, it was found that crack could appear on the surface of the film grown after the growth of the laminate structure, thereby potentially causing the yield to be lowered during the production stage.
Generation of the above problems is caused by the crystalline defect, i.e., the “dislocation” generated in the above nitride composite semiconductor laser element. It was confirmed that, normally, the crystalline defect was generated on the surface of the GaN composite substrate by approximately 5×107 cm−2. By applying any effective means for bending or extinguishing the crystalline defect, it is possible to generate such a region containing crystalline defect with a low density, thereby enabling to secure a sufficient service life under a high-output condition in which technical problems still remain without being solved. It is further suggested that, by providing a mechanism capable of structurally relaxing strain inside of crystal layers formed on the GaN composite substrate, it is possible to lower the probability of causing crack to be generated, thereby preventing the yield rate from being lowered otherwise caused by the crack generated on the surface of the grown film.